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Fisher 500-C/MPX-65 consult needed for 19KHz amp bias

Discussion in 'Tuners' started by chazix, Oct 31, 2017.

  1. chazix

    chazix AK Subscriber Subscriber

    Messages:
    1,160
    Location:
    Groton MA
    Hoping that thread title whets the interest of some of the real wizards...

    Fisher 500-C with MPX-65 stereo decoder is in (putatively) just for alignment. S/M here:

    https://www.hifiengine.com/hfe_downloads/index.php?fisher/fisher_500-c_service_10001-19999_en.pdf

    Unit is in pretty good shape and has tuned up pretty nicely save for one thing: Stereo sound quality goes to hell at high-ish (but well within normal range) levels of baseband modulation.

    It's clear that at least one aspect of the problem is that the 19KHz amp (half of V100, a 12AX7) is going into hard cutoff when baseband modulation hits peaks. Thus, the drive to the 38KHz section gets weak to non-existent, and decoding stops, during those peaks.

    The bias for the 19KHz amp seems to be designed to vary in response to some high-pass-filtered component of the the input to the MPX section - reference the neighborhood of the semiconductor rectifier, CR100. But I'm not sure what the designers intended here. Why should the bias vary with input signal at all - and what particular component of the input is supposed to determine the bias?

    Thanks for any insight!

    chazix
     

     

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  2. gadget73

    gadget73 junk junkie Subscriber

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  3. chazix

    chazix AK Subscriber Subscriber

    Messages:
    1,160
    Location:
    Groton MA
    Thanks, good advice. As it turned out, I didn't even have to post, just look around. The circuitry I was uncertain about acts as a noise gate, disabling the 19KHz amp if there's too much above-band content in the MPX input. This was described, and a modification suggested, here:

    http://audiokarma.org/forums/index....ers-and-receivers.708033/page-4#post-10535956

    The modification initially struck me as being a bit radical, but I tried it as described, and I liked the results. Thanks and a tip of the hat to @dcgillespie !

    chazix
     
  4. gadget73

    gadget73 junk junkie Subscriber

    Messages:
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    Location:
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    Cool. I figured Dave would probably have known the answer.
     

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